It's not possible to download big files, usually files bigger 500 MB, from the cPanel account when accessed via service subdomain "CPANEL.domain.com"
This issue is related to LiteSpeed. It is indeed LiteSpeed that's aborting the download. LiteSpeed sets a Max Request Body at 500MB by default - but it can be changed.
grep -Ei "maxReqBodySize" /usr/local/lsws/conf/httpd_config.xml
That value needs to be changed. After changing that value you should restart the service with this command: